1. Field of the Invention
This invention pertains generally to the control of the flow of gas, and more particularly, to a system which includes an improved diaphragm type gas flow controller.
2. Description of the Prior Art
Precise delivery of gas, the gas flow rate (volume per unit time), is critical to the operation of many laboratory instruments such as gas chromatographs, gas calibration units, and headspace sampling systems. Perhaps the most simple method of adjusting gas flow is by holding the upstream pressure constant against a variable orifice in the gas stream, for example, a needle valve or other type of metering valve, or conversely, changing the upstream pressure against a fixed restriction in the gas stream. If, however, the downstream pressure varies due to changes in downstream restriction or temperature, the gas flow will not remain constant.
Various types of flow controllers have been developed to compensate for changes in downstream pressure by maintaining a constant differential pressure across a restriction integral to the controller or by sensing changes in gas flow and operating a metering valve in the gas stream to compensate for these changes and thereby sustaining a constant flow rate. There are presently three major methods for maintaining constant gas flow for instrumentation.
Probably one of the oldest device is the diaphragm flow controller where upstream and downstream pressure exert an opposing force on a diaphragm. Movement of the diaphragm under these forces opens and closes a valve or nozzle, whose reference position is established by a spring force. Supplying the gas to the downstream sides of the diaphragm establishes a differential pressure across an orifice or restriction in the gas path between the upstream and downstream sides of the diaphragm. If the downstream pressure rises, the diaphragm will move against the spring force until the pre-set differential pressure is reestablished. This gas control method is quite robust and stable over time, but is dependent on a constant upstream gas pressure.
The second type of apparatus for maintaining a constant gas flow is the mass flow controller, where gas flow is sensed by the transfer of heat from an electrically heated element to another element which is part of a resistance bridge or in an even simpler version, where a resistive element changes temperature under the influence of a flowing gas removing heat from that element. In either case, the sensed change in gas flow can, with appropriate amplification of the electrical signal, be used to open or close an electrically operated valve or restrictor to maintain constant gas flow against upstream or downstream changes in gas pressure.
A third apparatus for maintaining a constant gas flow utilizes an electrical sensor to determine the differential pressure across an orifice and to adjust the orifice or valve to deliver a preset differential pressure. Because gas flow is proportional to the square root of differential pressure across an orifice or restriction (by Bernoulli's equation), such a device can be utilized with appropriate factors for individual gases to translate differential pressure directly into gas flow.
These last two methods for controlling gas flow are capable of not only controlling the gas flow but also of yielding an electrical signal that may be used to indicate the magnitude of the gas flow. On the other hand, the diaphragm controller must utilize an external device to measure the gas flow which is set by the spring force against the diaphragm. This force could, of course, be supplied by a load cell integral to the diaphragm controller and the electrical signal could thus be translated by appropriate circuitry into an indication of flow rate. In practice, however, most users of diaphragm flow controllers measure the gas flow with such devices as rotometers, turbine meters, soap film meters, or the like.
An advantage of the diaphragm flow controller not shared by the other two devices is the robust character of a strictly mechanical device. However, the devices used to measure the gas flow, such as the rotameter, bubble meter, and mass flow meter, tend to be inaccurate primarily because they require constant recalibration.
The mass flow controller and the differential pressure sensor, although they do not require gas flow measurement devices, tend also to drift away from accurate calibration due to changes in the electrical characteristics of the sensors with time.
A common fault in all three gas flow controllers is the recalibration required each time a different type of gas is monitored or the monitoring conditions vary.
Therefore, it is an object of the present invention to provide a system having a diaphragm type gas flow controller that does not require constant recalibration or the use of external gas flow measurement devices, is impervious to both the upstream and downstream pressure changes, and automatically accommodates changes in the gas flow being controlled.